The demand on an electric grid may vary greatly on a day to day basis and even on an hour to hour basis. These variations may be particularly true in geographic regions with a significant percentage of renewables such as wind, solar, and other types of alternative energy sources. Overall gas turbine and power plant efficiency, however, generally requires gas turbine operation at base loads. Any reduction from base load may not only reduce efficiency but also may decrease component lifetimes and may increase undesirable emissions.
Nonetheless, there is a commercial need for spinning reserves to accommodate this variation in the load on the grid. Given such, there is a desire for traditional generating units to have “hibernation” capacity. That is, a generating unit is online but operating at an extremely low power, output, i.e., extreme turndown loads. Such an operating mode is largely inefficient because valuable energy in the compressor air flow is discharged as bleed air and as such may be wasted. Moreover, compressor stall or surge may be a risk.
Current generating units may be limited to a hibernation mode of approximately forty-five percent (45%) or so of base load for an extended duration. Any further turndown may result in inadequately cooled turbine stage buckets as well as possibly exceeding component operating constraints, i.e., “a pinch point” in later turbine stages. Specifically, mechanical property limits, operational parameter limits, and emission limits may have an impact on the overall turndown percentage that may be reached safely.
There is thus a desire for improved gas turbine cooling systems so as to provide adequate cooling even during extreme turndown operations without the loss of overall efficiency, a decrease in component lifetime, or an increase in undesirable emissions. Moreover, the gas turbine engine should maintain the ability to ramp up quickly to base load when needed.